The long-term objectives of this study are to assess the histologic damage and efficacy of ablation of cardiac arrhythmic foci by means of cardiac electroshock. We propose to deliver shocks of varying intensity to different areas of the atria and ventricles and assess the histologic damage. The shocks will be delivered through a standard direct-current defibrillator from an electrode catheter placed in various areas of the heart to a plate on the chest wall. We will also determine change in threshold in different areas of the heart before and after the delivered shocks. We propose to use a canine-infarction model to produce ventricular tachycardia. These dogs will undergo endocardial mapping for tachycardia localization, and attempted ablation by means of one or more delivered shocks. The purpose of these studies are to hopefully extend these procedures for use in patients with cardiac arrhythmias who are refractory to drug treatment.